Millisecond pulsars ( MSPs ) orbiting helium white dwarfs ( WDs ) in eccentric orbits challenge the established binary-evolution paradigm that predicts efficient orbital circularization during the mass-transfer episode that spins up the pulsar . ( 19 ) recently proposed that these binary MSPs may instead form from the rotationally delayed accretion-induced collapse of a massive WD .
This scenario predicts that eccentric systems preferably host low-mass pulsars and travel with small systemic velocities—in tension with new observational constraints .
Here I show that a substantial growth in eccentricity may alternatively arise from the dynamical interaction of the binary with a circumbinary disk .
Such a disk may form from ejected donor material during hydrogen flash episodes , when the neutron star is already an active radio pulsar and tidal forces can no longer circularize the binary .
I demonstrate that a short-lived ( 10 ^ { 4 } -10 ^ { 5 } yr ) disk can result in eccentricities of e \simeq 0.01 - 0.15 for orbital periods between 15 and 50 days .
Finally , I propose that , more generally , the disk hypothesis may explain the lack of circular binary pulsars for the aforementioned orbital-period range .